Acclimation to water scarcity causes the physiological condition of house mice (Mus musculus) and of spiny mice (Acomys cahirinus) to differ in degree and kind from the condition of control animals. The purpose of this project is to study the role of arginine vasopressin (AVP) and the hypothalamic-neurohypophyseal system (HNS) in the mediation and support of acclimation. Evidence, from our previous studies of acclimation and from the literature, suggests the hypothesis that during acclimation the HNS increases its general level of activity and possibly develops long-term functional modifications. Two experiments will test this hypothesis: 1. Comparing acclimated and non-acclimated animals with respect to plasma and neurohypophyseal AVP. 2. Determining if the threshold and sensitivity of the AVP-osmolality reflex are altered as a consequence of acclimation. Both experiments require the development and validation of a radioimmunoassay for AVP.